1. Field of the Invention
The present invention relates to an improvement in the method for attaching leads of electronic components to a printed circuit base board and to a printed circuit base board that can be advantageously used for working this improved method.
2. Description of the Prior Art
In general, a printed circuit base board is formed by attaching electrically conductive thin strips for connecting components of an electronic appliance to one surface of an electrically insulating substrate, and printed circuit base boards of this type are industrially manufactured by the copper laminate etching process (for example, the photoresist method, the silk screen method, the offset method, the rubber plate method and the electrophotographic method). Resistors, capacitors, transistors, diodes and other electronic components of electronic appliances are attached to this printed circuit base board according to an electric circuit pattern formed on one surface of the base board.
Components of electronic appliances have heretofore been attached to these printed circuit boards according to the procedures illustrated in FIGS. 1 and 2. More specifically, bores 4 are formed in the printed circuit base board 1 comprising an electrically insulating substrate 2 and a copper foil 3 laid out on one surface of the substrate 2 according to a desired electronic circuit pattern, at component attachment positions A so that leads 5 of the electronic appliance components (referred to as "component leads" in the instant specification and appended claims) can be inserted into these bores 4 and soldered to the printed circuit base board 1 as thus inserted, and component-attaching lands 3a are formed on the peripheral edges of one opening 4b of these bores 4. Then, the component leads 5 are inserted into the bores 4 from the reverse surface of the printed circuit base board 1, namely from the other openings 4a of the bores 4 so that these component leads 5 project from the front surface of the printed circuit base board 1. Then, the projected portions 5a of the component leads 5 are soldered to the component-attaching lands 3a formed of the copper foil 3 around the openings 4b of the bores 4 on the front surface of the printed circuit base board 1.
The above-mentioned bores 4 are ordinarily formed by press processing or drilling. These bores are generally formed with certain dimension allowances for errors in the drilling operation, shrinkage of printed circuit base boards and deviations in the thickness of component leads owing to differences of types and manufacturers with due consideration to facilitation of the operation of inserting components leads into the bores.
Accordingly, as conceptually shown in FIG. 2, slight clearances or gaps are ordinarily formed between the side walls of the bores 4 and the component leads 5 when the leads 5 are inserted into the bores 4. Because of the presence of such clearances, in the conventional attachment method, the following disadvantages are brought about during or after the attachment operation.
In the practical attachment process line, the above-mentioned operations of insertion and soldering of component leads into the bores are performed by an automatic soldering machine. In this soldering process, because of the presence of these clearances or gaps and/or because of a gas generated from the insulating substrate by the action of the heat for the soldering operation, the solder is caused to partially fall out. Namely, a so-called tunnel phenomenon (formation of defects such as holes 6a shown in FIG. 3) often occur.
This tunnel phenomenon results in insufficient electric contact between the component leads and the component lead-attaching lands. Therefore, these defects should be eliminated by a correcting operation after the soldering step. Accordingly, in the above-mentioned automatic soldering operation, it is indispensable to check for formation of tunnel defects and perform a correcting operation for eliminating these defects, and these troublesome additional operations require much labor and time and hinder attainment of the labor-saving effect of the soldering process.
As means for eliminating the foregoing disadvantage, there have been proposed and utilized a one-surface bore method and a bore plating method. These methods, however, are not suitable for the manufacture of commercial circuits because the productivity is very low, and they are not advantageous from the economic viewpoint.
As pointed out hereinbefore, the presence of the gaps between the side walls of the bores and the component leads results in the above-mentioned insufficient electric contact between the component lead-attaching lands and the component leads attached thereto by the automatic soldering operation or the like. Still further, if the above-mentioned gaps are present, no complete attachment is attained by simple soldering and breakages of the leads are readily caused by external forces such as vibrations. Accordingly, no sufficient electric or functional reliability can be attained and the durability, for example, the vibration resistance, is poor.
Moreover, in the conventional attachment method, in order to assure a satisfactory soldering effect, a solder mask (resist) 7 should be formed on the land 3a for attachment of the component lead 5 while the entire surface of the land 3a is kept in the uncoated exposed state. Accordingly, the circuit density cannot be enhanced and it is impossible to obtain a compact printed circuit.